Abstract:

Many bats, including some small-bodied tropical and sub-tropical Pteropodidae, use torpor to offset energetic constraints. We tested the hypothesis that medium sized (110-160 g) cave-roosting Egyptian rousette bats (Rousettus aegyptiacus) at the southern extent of their range are able to employ torpor. We measured daytime body temperatures (Tb) of nine wild individuals using implanted temperature-sensitive radio-transmitters. The bats roosted in a cave on Table Mountain, Cape Town, South Africa (a typically cool and wet area). Daily mean cave temperature (Tc) ranged between 7 and 12 °C (mean 9.3 °C). All wild individuals exhibited a circadian cycle in Tb, with an average of 37.7 °C upon return from foraging, decreasing to 35.5 °C by mid-day. Before emergence for feeding, Tb increased to about 37 °C. No individual allowed Tb to drop below 34 °C indicating (assuming a threshold temperature of 30 °C) that individuals in this population do not use torpor. Bats were active throughout the day within the roost and metabolic heat production may have contributed to the maintenance of rest phase Tb. Ten individuals implanted with temperature sensitive passive integrated transponder tags were held in captivity at temperatures of 25-30 °C and subjected to food deprivation for 2-3 days. The lowest Tb recorded for any captive individual was 33.4 °C despite losing an average of 10% of their initial body mass. Despite exposure to cool Ta in the cave, and often cold, wet and windy conditions whilst foraging, or substantial food restriction in captive bats, R. aegyptiacus did not resort to using torpor.